1. Field of the Invention
The present invention relates to a vehicular active vibratory noise control apparatus for canceling a road noise by causing a canceling sound that is in opposite phase with the road noise to interfere with the road noise.
2. Description of the Related Art
Heretofore, there has been proposed in the art an active noise control (ANC) apparatus for canceling a road noise (also called “drumming noise”) in the passenger's compartment of a vehicle with a canceling sound that is in opposite phase with the road noise at an evaluating point (hearing point) where a microphone is located (see Japanese Laid-Open Patent Publication No. 2000-280831). The road noise is based on the vibrations of vehicle wheels which are caused by the road when the vehicle is running on the road, transferred through the suspensions to the vehicle body, and particularly excited by the acoustic resonant characteristics of the closed room such as a passenger's compartment. The road noise has a peak level at a frequency of about 40 [Hz] and has a frequency bandwidth in the range from 20 to 150 [Hz].
The vehicle has various rotating components including an engine crankshaft, a transmission main shaft, a transmission countershaft, a propeller shaft, etc. which rotate when the engine on the vehicle operates. The rotational frequency of these rotating components varies depending on the speed of the vehicle, etc. When these rotating components rotate, they produce a noise (hereinafter referred to as “engine muffled sound” to be distinguished from the road noise) in the passenger's compartment based on the rotational frequency.
It has been found that when the active noise control apparatus for canceling the road noise is turned on in a frequency range wherein the rotational frequency of the rotating components in the passenger's compartment overlaps the frequency of the road noise, the engine muffled sound caused by the rotation of the rotating components does not change per se, but tends to increase at the evaluating point.
For example, FIG. 12 of the accompanying drawings shows a graph of sound pressures (represented by the vertical axis) measured in the position (evaluating point) of the ears of the driver of a vehicle at different frequencies (represented by the horizontal axis). As shown in FIG. 12, a characteristic curve 2 indicated by the dotted lines is plotted when the road-noise ANC apparatus on the vehicle is turned off, and a characteristic curve 4 indicated by the dot-and-dash lines is plotted when the road-noise ANC apparatus on the vehicle is turned on. As indicated by the characteristic curves 2, 4, at the frequency of 42 [Hz] at which the sound pressure of the road noise is maximum, the sound pressure is lower by 10 [dB] or more when the road-noise ANC apparatus is turned on than when the road-noise ANC apparatus is turned off. However, at the frequency of 65 [Hz] which corresponds to the rotational frequency, the sound pressure is higher by about 5 [dB] when the road-noise ANC apparatus is turned on than when the road-noise ANC apparatus is turned off. The road-noise ANC apparatus is disadvantageous in that while it can reduce the road noise, it tends to increase the engine muffled sound at the frequency of 65 [Hz].
Furthermore, as shown in FIG. 13 of the accompanying drawings, if the rotational frequency is of about 45 [Hz], then a characteristic curve 6 indicated by the dotted lines is plotted when the road-noise ANC apparatus is turned off, and a characteristic curve 8 indicated by the dot-and-dash lines is plotted when the road-noise ANC apparatus is turned on. The comparison of these characteristic curves 6, 8 shows that the road-noise ANC apparatus is not effective to lower the sound pressure at the rotational frequency of about 45 [Hz].